Catalysts used in the conversion and refining of hydrocarbons, as well as desorbents or adsorbents in the hydrocarbon separation or purification industry have a desired particle length requirement.
Fresh, or virgin catalysts, desorbents and adsorbents require segmentation into a particular particle length category in order to achieve optimum packing or reduce fixed bed pressure drop, or in the case of fluid beds achieve optimum pumping requirements.
In addition, catalysts used in the conversion and refining of hydrocarbons become contaminated with carbonaceous deposits, known as "coke", as well as metallic deposits, such as nickel, vanadium, iron or silica which may be present in the hydrocarbon feed as organometallic compounds or as tramp impurities. The spent catalyst becomes inactive and must be withdrawn from the catalyst reactor, and passed to a regenerator where coke is burned off to regenerate the catalyst, which is returned for reuse in the reactor. In addition to contamination, continued use of the catalyst results in catalyst attrition, i.e., the generally cylindrical catalyst particles become reduced in length to an undesirable degree. The presence of such shortened catalyst particles in a fixed bed reactor causes plugging of the bed and undesirable pressure buildup resulting in reduced flow and eventual shutdown of the bed. Historically, the catalyst industry has utilized classical screening techniques to remove "dust" or "fines", i.e., very small broken or deformed catalyst particles, from the catalyst mass. However, use of a screen with a square aperture is limited by the diameter of the generally cylindrical catalyst particle or "pill". If the aperture of the screen is larger than the cross-sectional diameter of the pill, the screen cannot separate the catalyst pills on the basis of length. Screening can only be used to separate pills having a length-to-diameter ratio (L/D) less than one from pills having an L/D equal to or greater than one. Thus, a 12 mesh screen (square or slotted) having a 1.7 millimeter opening, which is either square or rectangular, will allow a 1/16 inch (1.6 mm) diameter cylindrically shaped catalyst particle through the opening, whether the catalyst is 2 millimeters or 10 millimeters in length, as long as the catalyst enters the opening on end. Accordingly, in practice, the use of screening techniques is limited to removal of catalyst dust or fines in which the catalyst L/D is less than one.
Unfortunately, catalyst particles having an L/D greater than one can still be too short for usage in the reactor. Most frequently it is the particles with an L/D in the range of 1.0 to 2.0 that are undesirable and cause problems necessitating disposal of the entire catalyst batch containing such particles. Not only is such disposal costly and wasteful, in view of the great expense of catalysts, including those containing noble metals and the like, but, in addition, such disposal results in pollution and toxic waste disposal problems since many such catalysts contain toxic metals, such as cobalt, nickel, and vanadium.
The manufacturing process for shaped catalysts includes sizing and/or breaking procedures to produce a product of a desired particle length distribution. With the state of the art in screening and separation technology, it is frequently difficult if not impossible to obtain the desired particle length distribution, or if it is achieved, this commonly results in the rejection of desired particles along with those undesired because of the poor selectivity of the equipment or process. Major improvements can be obtained if particles can be selectively separated on a length basis if the length division point is such that the L/D ratio is greater than one.
Additionally, spent catalyst particle mixtures are "heterogeneous" in nature since the degree of feedstock-supplied metal contamination on each particle varies within the batch. Thus, a given batch of spent catalyst contains good catalyst particles and contaminated catalyst particles in varying degrees. However, since adequate methods are unavailable to separate the good particles from the more contaminated particles, the entire batch is often discarded.
In other cases a given lot of catalyst may consist of two or more similarly shaped and sized particles but different in composition and individual pellet density. This may commonly be, for example, mixtures of hydrotreating and hydrocracking catalysts or nickel-molybdenum and cobalt-molybdenum catalysts. Such mixtures may be spent catalyst removed from a processing reactor or fresh catalyst which may have been inadvertently mixed during the manufacture or a handling process. These mixtures have diminished value and are commonly discarded.